A fuel cell, as presented in FIG. 1, includes an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. In solid oxide fuel cells (SOFCs) oxygen 106 is fed to the cathode side 102 and it is reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with fuel 108 producing water and also for example carbon dioxide (CO2). Anode 100 and cathode 102 are connected through an external electric circuit 111 which includes a load 110 for the fuel cell withdrawing electrical energy alongside heat out of the system. In electrolysis operating mode (solid oxide electrolysis cells (SOEC)) the reaction is reversed, i.e. heat, as well as electrical energy from a source 110, are supplied to the cell where water and often also carbon dioxide are reduced in the anode side forming oxygen ions, which move through the electrolyte material to the cathode side where de-ionization to oxygen takes place. It is possible to use the same solid electrolyte cell in both SOFC and SOEC modes. In such a case and in the context of this description the electrodes are often named anode and cathode based on the fuel cell operating mode, whereas in purely SOEC applications the oxygen electrode may be named anode.
Solid oxide electrolyser cells operate at temperatures which allow high temperature electrolysis reaction to take place, the temperatures being for example between 500-1000° C., but even over 1000° C. temperatures may be useful. These operating temperatures are similar to those conditions of the SOFCs. The net cell reaction produces hydrogen and oxygen gases. The reactions for one mole of water are shown below, with reduction of water occurring at the anode:H2O+2e−--->2H2+O2−  Anode:O2−--->½O2+2e−  Cathode:H2O--->H2+½O2.  Net Reaction:
FIG. 2 shows a SOFC device as an example of a high temperature fuel cell device.
SOFC device can utilize as fuel for example hydrocarbons such as natural gas or bio gas, alcohols such as methanol or even ammonia. The SOFC device in FIG. 2 includes more than one, for example plural fuel cells in stack formation 103 (SOFC stack). Each fuel cell can include anode 100 and cathode 102 structures as presented in FIG. 1. The SOFC device in FIG. 2 also includes a fuel heat exchanger 105 and optionally also a reformer 107. For example several heat exchangers are used for controlling thermal conditions at different locations in a fuel cell process. The reformer 107 is a reactor that may be used to convert feed stocks such as for example natural gas to a composition suitable for fuel cells, for example to a composition containing hydrogen and methane, carbon dioxide, carbon monoxide and inert gases. Since the process of reforming hydrocarbon fuel involves steam, it is beneficial to recover water formed as a product of fuel oxidation in fuel cells and to use the water for fuel reforming in the reformer, thus omitting a need for an external water feed to the system once the system is already operational and generating electricity. A practical method for recovering water formed as a product of fuel oxidation reactions in the fuel cell is anode off-gas recirculation in which a fraction of the gas exhausted from anode is recirculated to be mixed with unused feed stocks through a feedback arrangement 109.
By arranging anode exhaust gas recirculation at high temperature, it is also possible to omit at least one heat exchanger. While the primary purpose of anode gas recirculation in power generating mode is to ensure favorable gas composition at reformer inlet for facilitating desired reformation reactions, it also has a benefit of increasing overall fuel utilization within the bounds of desired level of single pass reactant utilization compared to a single pass operation alone. In an electrolysis operating mode, exhaust gas recirculation may also be used for conditioning cell inlet composition for optimal electrolysis performance at desired reactant utilization rate. In both operating modes, anode gas recirculation increases overall fluid flow through stacks, improving gas flow distribution and consequently temperature and composition distribution.
During start-up of a SOFC or SOEC system, recirculation of anode gas can serve a purpose of improving distribution of heat from heat sources throughout the system or, during shut-down, means for gas circulation may assist in flush dilution of the system. In case of partial oxidation (PDX) of fuel for maintaining required oxygen-to-carbon ratio, anode gas recirculation may serve a purpose of restraining otherwise high temperature increase in PDX reactor.
Accomplishing anode gas recirculation involves recovering a fraction of anode exhaust gas flow and re-pressurising it in essence to boost its pressure enough to overcome pressure losses in the recirculation loop at a given flow. In one known embodiment of the arrangement a high pressure fuel feed is used as a motive stream in a jet-ejector to entrain anode exhaust gas and to increase pressure of the entrained gas to the level of the fuel feed-in. Due to fixed geometry of the jet-ejector, these system topologies have a limited capability for controlling the re-circulation ratio and the resultant Oxygen-to Carbon (O/C) ratio at reformer or stack inlet and therefore can include compensating means such as an external water feed and a steam generator in the system to ensure adequate but not excessive steam content at fuel cell stacks. Insufficient steam flow rate can lead to disadvantageous and potentially irreversible formation of soot in the components throughout the fuel side of the system. On the other hand, excessive recirculation and consequent fuel gas dilution, while not as catastrophic to the system as insufficient recirculation, would lower fuel cell voltages and efficiency.
Another known embodiment of the arrangement is to accomplish desired recirculation pressure boosting by a fan or a compressor. Recirculation carried out by a fan or a compressor provides added flexibility and controllability to the system operation but involves sophisticated, complex and potentially unreliable machinery. Particularly accomplishing anode gas recirculation at very high temperature, which could provide advantages of simplifying the thermal integration scheme, sets difficult design requirements and potentially adds complexity to the fan or to the compressor.